In recent years, in wireless devices and the like, miniaturization and thinning of antennas have been required. This is caused by the fact that securing space is difficult to due to the high packaging density, an increase in the number of antennas due to introduction of a multiple input multiple output (MIMO), and the like. Such a tendency is remarkable, particularly, in mobile applications in which miniaturization, lighter weights, and thinning are required, and thus miniaturization and thinning of antennas are essential.
In resonator antennas such as a related art type patch antenna or a wire antenna, the operating band thereof depends on the element size, and the dielectric constant and the magnetic permeability of an insulating material (dielectric). Therefore, the operating band and the used substrate material are determined, the size thereof also is naturally determined.
FIG. 2 shows a related art type patch antenna 1a. It is constituted by two conductor layers. A patch-shaped conductor element 2 which is an antenna element is disposed in the upper layer and a conductor plane 3 is disposed in the lower layer with a dielectric layer 14 interposed therebetween, and a region surrounded by the dotted line forms a resonator 12. In addition, the conductor element 2 is electrically connected to a power feed line 6. In an example of the drawing, power is fed to the conductor element 2 by a microstrip line.
Generally, since the carrier frequency used in wireless devices is in a range of a few GHz or less, the size equivalent to a half-wavelength λ/2 in a vacuum is a few cm or so in a vacuum. Here, when the dielectric constant of the dielectric layer 14 is set to εr, and the magnetic permeability is set to μr, the length d of one side of the resonator 12 during half-wavelength resonance is expressed by the following expression.d=λ/(2·(εr·μr)1/2)
Therefore, in order to drastically miniaturize the related art type antenna, it is required to use a medium having an extremely high dielectric constant and magnetic permeability, and thus it costs too much.
On the other hand, in recent years, it is proposed to use the high-impedance surface (hereinafter, referred to as HIS) as a method for improving the low profile or directionality of the antenna. The HIS is also referred to as an artificial magnetic conductor (AMC). As a structure for implementing the HIS, a mushroom-type periodic structure 10 disclosed in Patent Document 1 is known. The mushroom-type periodic structure 10 is also known as one of the typical structures of an electromagnetic bandgap (EBG) structure.
Patent Document 1 discloses that while a normal conductor causes electromagnetic waves to be reflected in reverse phase, the mushroom-type periodic structure 10 causes electromagnetic waves in the vicinity of the bandgap frequency to be reflected in phase, and functions as a magnetic wall and suppresses propagation of the surface current in the bandgap frequency band of the mushroom-type periodic structure 10.
FIG. 3 shows a cross-sectional view of the mushroom-type periodic structure 10. The mushroom-type periodic structure 10 has a structure in which it is constituted by two conductor layers, the periodic array of conductor strips 4 is disposed on the upper layer, the conductor plane 3 is disposed on the lower layer, and each of the conductor strips 4 is electrically connected to the conductor plane 3 by a conductor post 5. As a shape of the conductor strip 4, a regular hexagonal shape or a square shape, and the like are proposed.
FIG. 4 (a) shows a patch antenna 11 disclosed in FIG. 11b of Patent Document 1. In the example shown in the drawing, the power feed line 6 passes through the dielectric layer 14 so as to be connected to the coaxial cable 16. The mushroom-type periodic structure 10 is disposed so as to surround the conductor element 2 which is an antenna element, whereby propagation of the surface current is suppressed. Thereby, it is known from Patent Document 1 and the like that unnecessary radiation from the end or the rear of the conductor plane 3 is suppressed, and that directionality or radiation efficiency of the antenna is improved.
FIG. 4 (b) shows a wire antenna 21 disclosed in FIG. 8b of Patent Document 1. The operating frequency of the antenna, that is, the resonance frequency of the resonator 12 and the frequency at which the mushroom-type periodic structure 10 functions as a magnetic wall are matched with each other, whereby it is possible to use the mushroom-type periodic structure 10 as a reflective plate functioning as a magnetic wall. It is known from Patent Document 1 and the like that when a normal conductor plane is used as a reflective plate of the antenna, it is required to set the conductor element 2 apart from the conductor plane 3 to a height of a quarter wavelength in order to enhance the radiation efficiency, and on the other hand, when the mushroom-type periodic structure 10 functioning as a magnetic wall is used as a reflective plate, the radiation efficiency is enhanced at the time of bringing the conductor element 2 close to the mushroom-type periodic structure 10, thereby allowing a lower profile to be obtained in the antenna.
Moreover, in the wire antenna 21, the propagation of the surface current is also suppressed by the mushroom-type periodic structure 10. Thereby, it is known from Patent Document 1 and the like that the unnecessary radiation from the end or the rear of the conductor plane 3 is suppressed, and that directionality or radiation efficiency of the antenna is improved.